Interface arrangements are known to be connected between an Alternating Current (AC) system, often denoted AC grid and a Direct Current (DC) system, like a High Voltage Direct Current (HVDC) system. Such an arrangement typically includes a converter, such as a voltage source converter, for conversion between AC and DC and having a DC side connected to the DC system and an AC side for coupling to the AC system. The arrangement also often includes a transformer having a primary side connected to the AC system and a secondary side for coupling to the converter. There is also an AC breaker between the primary windings of the transformer and the AC system in order to protect the converter through isolating the converter station from the AC system. The transformer may furthermore comprise auxiliary windings, which may be used for power supply, for instance of a plant or converter station where the interface arrangement is provided. Such power may be used in the powering of the plant such as the powering of cooling equipment for cooling the components of the converter.
In relation to a converter, which may be a cell-based multilevel converter, there may occur a number of faults that need to be taken care of.
One type of fault is an overvoltage on an AC bus between the converter and the transformer. Such a fault may be handled through providing surge arresters, for instance between the AC bus and ground.
In cell-based multilevel converters this solution is difficult to implement because of the use of cells.
Furthermore, the opening of the AC breaker for instance for isolating the interface arrangement from the AC system, may sometimes fail because the AC current may contain too high a DC component and thereby the current between the converter and the AC system may lack zero-crossings, which makes the AC breaker impossible to open.
One destructive overvoltage on the phase arms can be caused by a single-phase-to-ground fault at between the transformer and an AC terminal of the converter.
It is also possible that a DC pole to ground fault may occur, which can cause high currents to flow in diodes of the converter.
There exist some solutions for handling such faults.
WO 2010149217 does for instance describe a method for protecting a transformer at single-phase-to-ground faults. According to the document, when a ground fault is detected the ungrounded neutral on the secondary side of a transformer is connected to ground. In this way overvoltages on the secondary side can be reduced while coping with arresters having lower requirements and fast switching devices.
U.S. Pat. No. 6,411,529 describes a power plant control system in which rectifying diodes are replaced with controllable semiconductors. Because of this the reverse currents through the diodes due to faults can be blocked by controlling the controllable semiconductors.
There is therefore a need for an improvement in handling the above mentioned type of faults, especially in cell based multilevel voltage source converters.